1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a quantum interference effect semiconductor device utilizing the Aharonov-Bohm (A-B) effect and a method of producing the same.
The function of electronic apparatuses, such as computers has become increasingly complex, resulting in a demand for high speed semiconductor devices. There are proposed devices for answering the demand. One of the proposals is the quantum interference effect semiconductor device which can operate at an extremely high speed by using a quantum interference effect based on a wave property of electron in quantum electronics (quantum mechanics).
2. Description of the Related Art
Due to the wave property of electrons (electron wave), when electrons flow through two paths (channels) between two leads and an electric field is applied to one of the paths (or a magnetic field is applied to an area which is enclosed with the two paths) to influence the electron wave, a phase difference between the electron waves of electrons flowing the two paths is generated, causing the two electron waves to interfere with each other. As a result, the A-B effect appears as an interference effect in quantum mechanics (e.g., cf. S. Datta, "Quantum Devices", Superlattices and Microstructures, Vol. 6, No. 1, 1989, pp. 83-93).
There is a proposed quantum interference effect transistor (semiconductor device) using the A-B effect, as shown in FIG. 1, which uses a two dimensional electron gas in a GaAs/AlGaAs hetero-structure (see, e.g., U.S. Pat. No. 4,550,330 and Japanese Patent Application No. 3-26426). In this quantum interference transistor, a buffer GaAs layer 82, an undoped GaAs channel layer 84 and an n-type AlGaAs electron-supply layer 86 are epitaxially formed on a semi-insulating substrate (not shown) and are patterned to form a predetermined shape consisting of two lead portions and a ring-portion therebetween, as shown in FIG. 1. A source electrode 92 and a drain electrode 94 are formed on the AlGaAs layer 86 at the two lead portions, and a source region 93 and a drain region 95 are formed in the lead portions of the layers 86, 84 and 82 under the electrodes 92 and 94, respectively. A gate electrode 90 is formed on the AlGaAs 86 at a portion of the half-ring.
When the quantum interference transistor operates, a two-dimensional electron gas 88 is generated in the channel GaAs layer 84 in the proximity of the interface between the layers 84 and 86, and flows from the source region 93 to the drain region 95 through two paths of the ring portion. When a voltage is applied to the gate electrode 90, the electron wave of the electron gas 88 flowing through the path under the electric field of the gate electrode 90 is varied with the result that a phase difference between the electron waves flowing through the two paths occurs. Thus, the interference between the electron waves is dependent on the phase difference and varies the drain current. Therefore, since the phase difference between the electron waves is dependent on the voltage applied to the gate electrode 90, a modulation of the gate voltage realizes a modulation of the drain current.
In order to cause a sufficiently interference effect, it is necessary to prevent the electrons from being subjected to not only an inelastic scattering but also an elastic scattering. However, in the above-mentioned quantum interference transistor, it is very difficult to form the ring with a high degree of accuracy and shorten a distance between the source and drain regions. Furthermore, the curve paths for guiding the electron waves in the ring portion easily cause the elastic scattering therein, which weakens the interference effect. Accordingly, the modulation of the drain current of the quantum interference effect transistor due to the interference effect is very small.
There is another proposed quantum interference effect transistor (semiconductor device), as shown in FIG. 2, which has two straight paths (layers) for passing electron waves so as to prevent the elastic scattering (see, e.g., Japanese Unexamined Patent Publication (Kokai) No. 63, 93161, published on Jul. 31, 1989). In this case, on a semi-insulating GaAs substrate 96, a GaAs buffer layer 98, a lower AlGaAs layer 102, a first channel GaAs layer 106, a separation AlGaAs layer 108, a second channel GaAs layer 104 and an upper AlGaAs layer 100 are continuously and epitaxially formed, in which the first channel GaAs layer 106 is a quantum well formed by sandwiching it between the lower AlGaAs layer 102 and the separation AlGaAs layer 108, and the second channel GaAs layer 104 is also another quantum well formed by sandwiching it between the separation AlGaAs layer 108 and the upper AlGaAs layer 100. A source electrode 114 and a drain electrode 116 are formed on the upper AlGaAs layer 100 and then are heat-treated to form an n.sup.+ -type alloy-contact (source) region 110 and an n.sup.+ -type alloy-contact (drain) region 112 extending into the lower AlGaAs layer 102. A gate electrode 118 is formed on the upper AlGaAs layer 100 at a center between the electrodes 114 and 116.
In the quantum interference effect transistor having two double-hetero-junction structures for the two channels, an electron wave from the alloy-contact (source) region 110 is separated into two electron waves which flow through the first and second channel GaAs layers 106 and 104 and reach the alloy-contact (drain) region 112. When a voltage is applied to the gate electrode 118 to influence the electron waves, a phase difference between the electron waves flowing through the two channels occurs. Thus, the two electron waves having the phase difference therebetween interfere with each other at the alloy-contact (drain) region 112, to vary the drain current, depending on the phase difference. Since the GaAs channel layers 104 and 106 are straight, the elastic scattering is small. Nevertheless, the electron wave is subjected to the inelastic scattering in the alloy-contact regions (i.e., lead channel portions) 110 and 112, so that the two electron waves flowing through the two channel GaAs layers have a phase difference at a random value. Accordingly, although the control of the phase difference by a gate voltage of the gate electrode 118 is attempted, it is difficult to obtain meaningful interference information. Taking the above-mentioned facts into consideration, the electron wave which can vary the drain current by the interference effect is specified to be an electron wave which is not subjected to the inelastic scattering in the regions (lead channel portions) 110 and 112 with the result that the efficiency of the modulation of the drain current is greatly lowered.
Furthermore, another type of quantum interference effect transistor shown in FIG. 3 has been proposed. On a semiconductor substrate 120, a buffer semiconductor layer 122, a lower n-type AlGaAs layer 124, a channel GaAs layer 128 and an upper n-type AlGaAs layer 126 are formed to interpose the channel GaAs layer 128 between the AlGaAs layers 124 and 126. A GaAsAlGaAs hetero-junction structure induces two-dimensional electron gas as a carrier, in the channel GaAs layer 128. In the quantum transistor, an island-shaped AlGaAs separation layer 130 is provided within the channel GaAs layer 128 to separate a middle portion of the channel layer 128 into two parts, as shown in FIG. 3. A source electrode 134 and a drain electrode 136 are formed on the upper AlGaAs layer 126 above the ends of the channel layer 128. An n.sup.+ -type alloy-contact (source) region 135 and an n.sup.+ -type alloy-contact (drain) region 137 are formed by annealing method to extend into the buffer layer 122 under the electrodes 134 and 136, respectively. A gate electrode 132 is then formed on the upper AlGaAs layer 126 above the middle portion (i.e., the island-shape separation layer 130).
An electron wave flowing from the source region 135 is separated into two electron waves which flow through an upper part and a lower part of the middle portion of the channel GaAs layer 128 and combine into an electron wave flowing into the drain region 137. When a voltage is applied to the gate electrode 132 to influence the electron waves, a phase difference between the electron waves flowing through the upper part and the lower part appears.
In order to form the island-shape separation AlGaAs layer 130 and the channel GaAs layer 128 surrounding the layer 130, an AlGaAs layer for the separation layer 130 is grown (formed) on a half of the GaAs layer 128 and is selectively etched to grow (form) the island-shape separation AlGaAs layer 130, and then the remaining half of the GaAs layer 128 is formed over the former GaAs layer 128 and the AlGaAs layer 130. Where an etching step is interposed between the growing steps of AlGaAs and GaAs, the AlGaAs layer is exposed to the atmosphere, so that the surface of the AlGaAs layer is very easily oxidized and both of the AlGaAs layer and the GaAs layer are easily contaminated by absorption of carbon atoms. As a result, a large number of interface states caused by impurities are generated between the AlGaAs layer 130 and the GaAs layer 128 to trap and/or scatter electrons flowing through the channel GaAs layer 128, which prevents effective interference from occurring.
As mentioned above, in conventional quantum interference effect transistors it is difficult to generate a sufficient electron wave interference. Furthermore, although the interference of the electron wave occurs, such an interference effect is not effective to control the drain current.